Devices are known, which are used for controlling the operation of electric rotating machines, employed as electric power generators and/or electric motors.
Particularly, these control devices, commonly known as inverters, can control the operation of the electric machine by adjustment of the electrical parameters of power signals, when the latter are continuously supplied to the electric machine.
Nevertheless, the need is particularly felt in the field of controlling the operation of the electric machine when the rotating masses thereof are subjected to speed transients.
These transients occur after a physical interruption in the power distribution network, or after a temporary voltage reduction or possibly upon undesired or expected cut-off of power to the inverter.
Due to these transients, synchronism between the inverter and the rotating masses is particularly difficult to restore.
Particularly, during transients, speed variations in the machine may be either reduced due to internal friction or load resistance, or maintained or further enhanced due to the presence of external devices capable of transmitting a drive torque to the shaft of the electric machine.
The control of the machine is typically restored by an external machine stabilization and/or shutdown action, which requires a relatively long time.
Therefore, these methods have a particularly penalizing effect on restoration costs for the plants and devices that use the transient-affected electric machines.
In an attempt to obviate this drawback, optimized control methods have been suggested for the particular type of the rotating electric machine controlled by the inverter.
If the electric machine is of asynchronous type, then the rotation speed of the rotating masses may be determined by injecting an appropriate voltage with a frequency falling within the operating range of the electric machine and by later detecting the sign of the resulting induced current.
However, if the electric machine is of synchronous type, the rotation speed of the rotating masses may be determined by very high superimposed voltages and currents which generate high acoustic noise at low frequencies. This is a particularly serious drawback if many electric machines are installed in the same environment, and may be subjected to speed transients, causing the emission of high acoustic noise.
Furthermore, in sensorless electric machines, the synchronization of the rotating masses requires very long times, generally a few seconds.
Alternatively, when the electric machine is of permanent-magnet or synchronous reluctance type, the angular position or the rotation speed of the rotating masses is detected using appropriate external sensors mounted to the drive shaft of the machine or integrated in the inverter.
A first drawback of this solution is that the provision of external sensors reduces the reliability of synchronous reluctance electric machines.
These sensors have wearing mechanical parts that cause frequent failures or require periodic replacement.
A further drawback of this solution is that the use of sensors increases the overall maintenance costs of the electric machine.
Furthermore, the replacement of sensors requires a temporary shutdown of the electric machine, thereby considerably reducing its overall efficiency.
Also, the use of sensors may add complexity to the construction of the inverter and increase the overall dimensions of the electric machine.